Interdigitated back contact silicon solar cells fabrication using diffusion barriers

ABSTRACT

Interdigitated back contact (IBC) solar cells are produced by depositing spaced-apart parallel pads of a first dopant bearing material (e.g., boron) on a substrate, heating the substrate to both diffuse the first dopant into corresponding first (e.g., p+) diffusion regions and to form diffusion barriers (e.g., borosilicate glass) over the first diffusion regions, and then disposing the substrate in an atmosphere containing a second dopant (e.g., phosphorus) such that the second dopant diffuses through exposed surface areas of the substrate to form second (e.g., n+) diffusion regions between the first (p+) diffusion regions (the diffusion barriers prevent the second dopant from diffusion into the first (p+) diffusion regions). The substrate material along each interface between adjacent first (p+) and second (n+) diffusion regions is then removed (e.g., using laser ablation) such that elongated grooves, which extend deeper into the substrate than the diffused dopant, are formed between adjacent diffusion regions.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/207,446, entitled “Interdigitated Back Contact Silicon Solar CellsWith Laser Ablated Grooves” filed Sep. 9, 2008.

FIELD OF THE INVENTION

This invention relates to the conversion of light irradiation toelectrical energy using photovoltaic devices (solar cells), moreparticularly, to methods and tools for producing interdigitated backcontact (IBC) solar cells, and to the IBC solar cells produced by thesemethods.

BACKGROUND OF THE INVENTION

Solar cells are typically photovoltaic devices that convert sunlightdirectly into electricity. Solar cells typically include a semiconductor(e.g., silicon) wafer (substrate) that absorbs light irradiation (e.g.,sunlight) in a way that creates free electrons, which in turn are causedto flow in the presence of a built-in field to create direct current(DC) power. The DC power generated by several solar cells may becollected on a grid placed on the cell. Solar cells are typically madeusing square or quasi-square silicon wafers that are doped to includeone or more n-type doped regions, and one or more p-type doped regions.Such solar cells (also known as silicon wafer-based solar cells) arecurrently the dominant technology in the commercial production of solarcells, and are the main focus of the present invention.

A desirable solar cell geometry, commonly referred to as theinterdigitated back contact (IBC) cell, consists of a semiconductorwafer, such as silicon, and alternating lines (interdigitated stripes)of p-type and n-type doping. This cell architecture has the advantagethat all of the electrical contacts to the p and n regions can be madeto one side of the wafer. When the wafers are connected together into amodule, the wiring is all done from one side. Device structure andfabrication means for this device have been described previously inco-owned and co-pending U.S. patent application Ser. No. 11/336,714entitled “Solar Cell Production Using Non-Contact Patterning andDirect-Write Metallization”, which is incorporated herein by referencein its entirety. On May 12, 2008, SunPower Corp. (San Jose, Calif., USA)announced achieving 23.4% efficiency in a prototype IBC cell (seehttp://investors.sunpowercorp.com/releasedetail.cfm?ReleaseID=309613).

A problem with IBC solar cells is that the conventional fabricationprocess used to produce IBC cells is quite complicated and, hence, moreexpensive as compared to the fabrication processes require to produceconventional ‘H-pattern’ solar cells. According to D. H. Neuhaus and A.Munzer, “Industrial Silicon Wafer Solar Cells” (Advances inOptoelectronics, vol. 2007, pp. 1-15, 2007), IBC cells require seventeenprocess steps (minimum) in order to complete the cell fabricationprocess, whereas conventional H-pattern solar cells require only ninesteps.

What is needed is a method for producing IBC-type solar cells thatovercomes the deficiencies of conventional production methods byreducing the manufacturing costs and complexity, whereby IBC-type solarcells can be produced at substantially the same or lower cost asconventional H-pattern solar cells.

SUMMARY OF THE INVENTION

The present invention is directed to a method for fabricating IBC solarcells that includes combining phosphorus and boron diffusion processesin which a screen-printable or spin-on-dopant boron source is depositedon the rear surface of a crystalline silicon substrate, and thenphosphorus dopant is diffused such that the boron source acts as adiffusion barrier for phosphorus diffusion to prevent a cross doping ofp+ and n+ diffusion regions. After the diffusion process, p+ and n+diffusion regions are separated by laser ablation, forming grooves inthe rear surface between the adjacent p+ and n+ diffusion regions. Theresulting fabrication process reduces the number of processing steps byapproximately half (in comparison to conventional methods), andfacilitates producing IBC solar cells at approximately the same (orlower) cost as currently required to produce ‘H-pattern’ solar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is a perspective view showing a partially fabricated IBC solarcell according to an embodiment of the present invention;

FIG. 2 is a cross-sectional side view showing the IBC solar cell of FIG.1 in a substantially completed state;

FIG. 3 is a flow diagram depicting a method for fabricating IBC solarcells according to another embodiment of the present invention; and

FIGS. 4(A), 4(B), 4(C), 4(D), 4(E), 4(F), 4(G), 4(H), 4(I), 4(J) and4(K) are cross-sectional side views showing an IBC solar cell duringvarious stages of the fabrication process of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to an improvement in photovoltaic devices(e.g., solar cells) that can be used, for example, to convert solarpower into electrical energy. The following description is presented toenable one of ordinary skill in the art to make and use the invention asprovided in the context of a particular application and itsrequirements. As used herein, directional terms such as “upper”,“lower”, “side”, “front”, “rear”, and “vertical” are intended to providerelative positions for purposes of description, and are not intended todesignate an absolute frame of reference. Various modifications to thepreferred embodiment will be apparent to those with skill in the art,and the general principles defined herein may be applied to otherembodiments. Therefore, the present invention is not intended to belimited to the particular embodiments shown and described, but is to beaccorded the widest scope consistent with the principles and novelfeatures herein disclosed.

FIG. 1 is a partial perspective view and FIG. 2 is a cross-sectionalside view showing a simplified IBC solar cell 100 according to anembodiment of the present invention. Those skilled in the art willrecognize that FIGS. 1 and 2 are simplified to show only a few diffusionlines and utilizes a distorted scale in order to highlight key featuresof the invention.

Referring to FIG. 1, IBC solar cell 100 is formed on a semiconductorsilicon (Si) wafer (substrate) 101 having a rear surface 103 and anopposing front surface 105. Substrate 101 includes several diffusionregions that are indicated in FIG. 1 by shading, and unshaded portionsof substrate 101 represent standard semiconductor Si. In one embodiment,the semiconductor Si is an n-type monocrystalline wafer with theresistivity between 0.1 and 2000 Ω·cm, but other type Si materials, suchas p-type monocrystalline Si wafer, and n-type or p-typemulticrystalline Si wafers, can also be used. Similar to conventionalIBC solar cells, IBC solar cell 100 includes multiple interdigitated(parallel, spaced-apart) diffusion regions 101-11 to 101-14 and 101-21to 101-23 that are formed through rear surface 103, and a continuousblanket (fourth) diffusion region 101-3 that is formed through frontsurface 105. A first set of diffusion regions 101-11 to 101-14 include ap-type dopant (e.g., boron) having a sheet resistance between 20 and 200Ω/square, and a second set of diffusion regions 101-21 to 101-23 includea n-type dopant (e.g., phosphorus) having a sheet resistance between 20and 200 Ω/square. The p-type dopant is diffused into substrate 101 suchthat diffusion regions 101-11 to 101-14 have a nominal depth D1 between0.1 and 5 μm, measured from rear surface 103 as indicated in FIG. 1, andhas a width W1 in the range of 100 to 3000 μm. The n-type dopant isdiffused into substrate 101 such that diffusion regions 101-21 to 101-23have a nominal depth D2 of 0.1 to 5 μm, measured from rear surface 103as indicated in FIG. 1, and has a width W2 in the range of 10 to 500 μm.The diffusion regions are arranged such that each of the second set ofdiffusion regions 101-21 to 101-23 is disposed between a correspondingpair of diffusion regions of the first set. For example, a (third) n+diffusion region 101-22 is disposed between a (first) p+ diffusionregion 101-12 and a (second) n+ diffusion region 101-13.

According to an aspect of the present invention, a series of grooves107-1 to 107-6 that are defined into rear surface 103 between adjacentpairs of the diffusion regions. Grooves 107-1 to 107-6 representelongated voids or openings in rear surface 103 where substrate materialis removed from between adjacent diffusion regions. For example, p+diffusion region 101-12 is separated from n+ diffusion region 101-22 by(third) groove 107-3, and p+ diffusion region 101-13 is separated fromn+ diffusion region 101-22 by a groove 107-4. Each groove 107-1 to 107-6has a (third) depth D3 (e.g., preferably in the range of 0.2 to 10 μm,and more preferably in the range of 0.5 to 1.5 μm) extending intosubstrate 101 from rear surface 103 that is greater than depths D1 andD2, whereby each adjacent pair of diffusion regions are physicallyseparated from each other by a corresponding groove (e.g., diffusionregions 101-12 and 101-22 are separated by groove 107-3).

In accordance with another aspect of the present invention, grooves107-1 to 107-6 are formed such that each diffusion region extendscontinuously between corresponding (vertical) side walls of two adjacentassociated grooves. That is, each groove has a width W3 (i.e.,preferably in the range of 1 to 50 μm, and more preferably in the rangeof 1 to 10 μm) defined by a distance between opposing vertical sidewalls (e.g., groove 107-1 has a width W3 measured between side wallsSW11 and SW12). Each diffusion region extends between associated sidewalls of adjacent grooves. For example, diffusion region 101-12 extendsbetween side wall SW42 of groove 107-4 and side wall SW51 of groove107-5. As set forth below, grooves 107-1 to 107-6 are formed such thatthe entire region between side walls SW42 and SW51 to the depth D1 hasthe p-type dopant (e.g., boron) that forms diffusion region 101-13.

IBC solar cell 100 is shown in a substantially completed state in FIG.2, wherein a surface passivation layer 120-1 (e.g., one of SiN_(x),SiC_(x), SiO₂, SiO₂/SiN_(x), or any other suitable dielectric materials)is formed over rear surface 103, and an anti-reflection layer 120-2(e.g., SiN_(x), TiO₂, or any other suitable dielectric materials) isformed over front surface 105, whereby diffusion regions 101-11 to101-14, 101-21 to 101-23 and 101-3 are protected by the respectivelayers. In accordance with another aspect of the invention, grooves107-1 to 107-6 are formed such that portions of passivation layer 120-1are respectively disposed in each groove. Metal contacts 130-11 to130-14 (e.g., AgAl) extend through passivation layer 120-1 and arerespectively connected to p+ diffusion regions 101-11 and 101-14, andmetal contacts 130-21 to 130-23 (e.g., Ag) extend through passivationlayer 120-1 and are respectively connected to n+ diffusion regions101-21 and 101-23.

FIG. 3 is a flow diagram depicting a method for fabricating IBC solarcells according to another embodiment of the present invention. FIGS.4(A) to 4(K) are simplified cross-sectional side views depicting theprocess of FIG. 3.

Referring to the top of FIG. 3 (block 205) and to FIG. 4(A), siliconwafer 101 is wet processed to facilitate surface texturing and cleaningon rear surface 103 and front surface 105.

Next, referring to block 210 of FIG. 3 and to FIG. 4(B), a p-type dopantsource (e.g., a printable Boron paste) is then deposited on rear surface103 in strips having a width in the range of 100 to 3000 μm, and morepreferably in the range of 1000 to 1200 μm, and with a spacing in therange of 10 to 500 μm, and more preferably in the range of 200 to 300μm. In one embodiment, the deposition of the p-type dopant sourceincludes an extrusion process such as that described in co-owned andco-pending U.S. Patent Application No. 20080138456, entitled “Solar CellFabrication Using Extruded Dopant-Bearing Materials”, which isincorporated herein by reference in its entirety. In another embodiment,the deposition of the p-type dopant source includes a well knownprinting process, such as screen printing, pad printing, or jetprinting. As indicated in FIG. 4(B), the resulting dopant material pads210-1 to 210-4 are disposed over regions 101-11A to 101-14A,respectively, which at this point in time are substantially undoped. Adrying process is then performed to dry the dopant material beforediffusion.

Referring to block 220 of FIG. 3 and to FIG. 4(C), diffusion of theboron into substrate 101 is then performed by placing substrate 101 intoa preheated POCl₃ furnace with the POCl₃ source turned off, the furnacetemperature to 900-950° C. to promote boron diffusion through rearsurface 103, thereby forming diffusion regions 101-11B to 101-14B. Inaddition, in accordance with another aspect of the invention, thetemperature during the boron diffusion process is selected such that theboron source material forms borosilicate glass layers 210-1A to 210-4Aon rear surface 104 over diffusion regions 101-11B to 101-14B. Notethat, after the boron diffusion process, diffusion regions 101-11B to101-14B are separated by substantially undoped regions 101-21A to101-23A.

Referring to block 225 of FIG. 3 and to FIG. 4(D), diffusion ofphosphorus (n-type dopant) into substrate 101 is then performed bycooling the POCl₃ furnace from the boron diffusion temperature (i.e.,900-950° C.) to a temperature in the range of 850-900° C. and thenturning on the POCl₃ at a rate sufficient to achieve the phosphorusdoping profile described herein. As indicated in FIG. 4(D), phosphorus(indicated by dashed line arrows) enters substrate 101 through frontsurface 105 and through exposed portions of rear surface 103, therebyforming n-type diffusion regions 101-21B to 101-23B and 101-3. Inaccordance with an aspect of the present invention, borosilicate glasslayers 210-1A to 210-4A serve as diffusion barriers during thephosphorus diffusion process to prevent diffusion of phosphorus intodiffusion regions 101-11B to 101-14B. The inventors believe that bylowering the furnace temperature below 900° C., the borosilicate glassformed during boron diffusion can effectively serve as a barrier forphosphor diffusion. Note that, at the end of the phosphorus diffusionprocess shown in FIG. 4(D), each adjacent diffusion region abuts itsadjacent diffusion region (e.g., diffusion region 101-12B abutsdiffusion region 101-22B at interface IF1, and diffusion region 101-22Babuts diffusion region 101-13B at interface IF2).

Referring to block 230 of FIG. 3 and to FIG. 4(E), grooves 107-11 to107-16 are then formed in rear surface 103 at each interface betweenadjacent diffusion regions, whereby each groove separates adjacent p+and n+ diffusion region pairs. In accordance with an aspect of thepresent invention, the groove formation process is performed by laserablation using such as a Q-switched solid state laser with a pulseenergy in a range such as from about 10 μJ to about 300 μJ wherebygrooves are formed having a depth of approximately in the range of 0.5μm to 5 μm and a width in the range of 5 to 50 μm.

Referring to block 240 of FIG. 3 and to FIG. 4(F), glass removal is thenperformed to remove remaining borosilicate glass pads 120-11 to 120-14(shown in FIG. 4(E). In one embodiment, glass removal is performed usinga wet chemical solution according to known techniques such as acid wetetching.

As shown in FIG. 3 (blocks 250 and 255) and in FIGS. 4(G) and 4(H), aSiNx anti-reflection layer 120-3 is then deposited in a PECVD reactor onfront surface 105 over diffusion region 101-3, and then a surfacepassivation layer (e.g., SiNx, SiCx, SiO2/SiNx) is deposited on rearsurface 103 according to known techniques such as PECVD or sputtering.

Referring to the lower portion of FIG. 3 and to FIGS. 4(I) and 4(J),AgAl paste portions 130-11A to 130-14A are respectively disposed onpassivation layer 120-1 over p+ diffusion regions 101-11 to 101-14(block 260 and FIG. 4(I)), and Ag paste portions 130-21A to 130-23A arerespectively disposed on passivation layer 120-1 over n+ diffusionregions 101-21 to 101-23 (block 265 and FIG. 4(J)). In an alternativeembodiment, Ag paste is deposited on both p+ diffusion regions 101-11 to101-14 and n+ diffusion regions 101-21 to 101-23 simultaneously using,for example, screen printing or extrusion, thereby eliminating oneprocess step and reducing the entire fabrication process to ten steps.Subsequent to the paste deposition (see block 270 of FIG. 3 and FIG.4(K)), substrate 101 is heated in a belt furnace to inducemetallization, whereby metal contacts 130-11 to 130-14 are formedthrough passivation layer 120-1 to p+ diffusion regions 101-11 to101-14, and metal contacts 130-21 to 130-23 are formed throughpassivation layer 120-1 to n+ diffusion regions 101-21 to 101-23.

An advantage of the present invention is that IBC solar cell 100(FIG. 1) can be fabricated with only ten process steps, which is sevensteps less than conventional IBC cell fabrication processes (i.e., asdescribed in D. H. Neuhaus and A. Munzer, “Industrial Silicon WaferSolar Cells” (Advances in Optoelectronics, vol. 2007, pp. 1-15, 2007)),and only one step more than the fabrication process typically used toproduce conventional ‘H-pattern’ solar cells. In additions, because thepresent invention enables the formation of IBC cells, the cost of moduleassembly, which accounts for 30-35% of the total solar cell module cost,is reduced by up to 30% over conventional “H-pattern” cell assembly,indicating that a 9.0-10.5% reduction in total module cost is possible,according to E. V. Kerschaver and G. Beaucarne, “Back-contact SolarCells: Review,” Progress in Photovoltaics: Research and Applications,vol. 14, pp. 107-123, 2006. Moreover, as the bow of wafers, which isusually caused by depositing Al paste and forming Al BSF (back surfacefield) on the entire rear surface in conventional “H-pattern” cells, canbe greatly reduced or even eliminated for IBC cells, it is much easierto use thin Si wafers to produce IBC solar cells, which will also reducethe cost of Si materials. With these cost reductions both on moduleassembly and Si materials, the present invention (which just has minorprocess cost increase comparing to conventional “H-pattern” cells)reduces the final cost by up to about 20%, comparing to conventionalcells even without efficiency improvement. Further, cost reduction to30% are potentially realized because, in general, IBC cells have higherefficiency than the conventional “H-pattern” cells. The cost/efficiencyanalysis is provided in Table 1 (below).

TABLE 1 Cost and Efficiency Analysis No Base process High η More η PARCI PARC II Si 100 100 100 100  75  75 Process 100  0 100 100 120 120Module 100 100 100 100  70  70 Eff. (%)  17.0  17.0  20.0  24.3  17.0 19.4 Power (W)   4.14  4.14   4.87   5.91   4.14   4.73 $/W   2.50 2.13   2.12   1.75   2.00   1.75 Cost NA  15%  15%  30%  20%  30%reduction

As indicated in the leftmost column of Table 1, the baseline process isprovided with costs of Si material, process, and module. Each costaccounts for 50%, 15%, and 35% of total module cost. The baselineprocess has 100% of Si material cost, 100% of processing cost, and 100%of module assembly cost, which result in total manufacturing cost of$2.50/W. Also, the cell/module efficiency is assumed to be 17%. The nextcost analysis (second column from left) investigates the manufacturingcost without any processing. Therefore, the processing cost is 0%.Assuming that the module is able to produce an efficiency of 17%, themanufacturing cost is $2.13/W, which represents 15% reduction in cost.The primary target of the present invention is to achieve 30% costreduction, so the “no process” option is not enough. The next costanalysis analyzes the impact of high-efficiency module, 20%. Assumingthe production of 20%-efficient modules without adding any processcompared to the baseline process, this option would produce amanufacturing cost of $2.12/W, which represents 15% reduction in cost. A20% efficiency module does not produce enough cost reduction. Therefore,the next step is to analyze the effect of an even higher cellefficiency, 24.3%, on cost. This higher efficiency module gives themanufacturing cost of $1.75/W, which represents 30% reduction in cost.However, achieving 24.3% without adding any process compared to thebaseline process is highly unlikely. The next step, PARC I, is theresult of cost analysis using the proposed device fabricationtechnologies. Our proposed IBC cells will be able to accept thin Siwafers, 100-150 μm thick. Therefore, the cost of Si material is reducedto 75% of its original value. The cell processing requires borondiffusion, laser ablation, and alignment/registration processes.Therefore, the processing cost is assumed to be increased 20%. Asdiscussed in the previous section, the cost of module assembly isreduced to 70% of its original value because the IBC structure providesa simpler module assembly process. Assuming that the cell/moduleefficiency is unchanged (17%), the manufacturing cost would be $2.00/W,which represents 20% reduction in cost. In order to achieve 30% costreduction using the proposed IBC cells, the cell/module efficiency needsto be improved to 19.4% from 17.0%, and this is very realistic for IBCcells. Hence, as demonstrated in Table 1, the present inventionfacilitates the production of IBC solar cells having a final costreduced by up to about 20% over conventional “H-pattern” cells (assumingthe same efficiency), and having a cost reduction of 30% or more whenthe higher efficiency of the IBC solar cells is taken into account.

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention. For example, although the presentinvention is described above with reference to n-type Si substrates, itis possible to start with p-type Si substrate. In this case, the widthof boron source would be in the range of about 10 to 500 μm, and morepreferably in the range of 200-300 μm, and the spacing would be in therange of 100-3000 μm, and more preferably in the range of 1000-1200 μm.In addition, the present invention is not necessarily limited to the useof boron and phosphorus as dopants (unless specified in the claims), andis intended to extend to any other dopants exhibiting the diffusionbarrier characteristics described herein, such as gallium (Ga) andarsenic (As). Moreover, the formation of grooves to separate the p+ andn+ diffusion regions is not necessarily limited to laser ablation, andmay be extended to any other suitable method capable of generating thegrooves described herein. For example, the grooves can also be made byusing selective chemical etching methods through such as printing orextruding an etching paste on the rear surface of the substrate.

The invention claimed is:
 1. A method of fabricating an interdigitatedback contact solar cell, the method comprising: diffusing a first dopantinto first and second substrate regions through a planar rear surface ofa semiconductor substrate such that the first dopant forms first andsecond spaced-apart diffusion regions in said first and second substrateregions having a first doping concentration and extending a first depthinto the substrate from the planar rear surface, and are separated by athird substrate region having a second doping concentration andextending a second depth into the substrate from the planar rearsurface, wherein diffusing the first dopant includes: depositingspaced-apart pads of material containing boron onto the planar rearsurface of substrate over said first and second substrate regions; andheating the substrate such that a first portion of the boron diffusesinto said first and second substrate regions through said planar rearsurface to form said first and second diffusion regions, and such that asecond portion of the boron forms spaced-apart first and secondborosilicate glass structures on said planar rear surface over saidfirst and second diffusion regions; and after forming said spaced-apartfirst and second borosilicate glass structures, diffusing a seconddopant into said third substrate region through said planar rear surfaceof said semiconductor substrate such that said second dopant passesbetween said spaced-apart first and second borosilicate glass structuresand forms a third diffusion region in said third substrate region, andsuch that said second dopant disposed in the third diffusion regionforms a first interface with the first dopant disposed in the firstdiffusion region and a second interface with the first dopant disposedin the second diffusion region, wherein said first and secondspaced-apart borosilicate glass structures form effective barriers thatprevent diffusion of said second dopant into said first and secondsubstrate regions during said diffusing of said second dopant.
 2. Themethod according to claim 1, further comprising, after forming saidfirst and second borosilicate glass structures, disposing said substratein a furnace containing POCl₃ at a temperature selected such thatphosphorus is diffused into said third substrate region and said firstand second borosilicate glass structures prevents diffusion of saidn-type dopant into said first and second substrate regions.
 3. Themethod according to claim 1, further comprising forming a plurality ofgrooves into the planar rear surface of the substrate such that thefirst diffusion region is separated from the third diffusion region by afirst groove, and the second diffusion region is separated from thethird diffusion region by a second groove.
 4. A method of fabricating aninterdigitated back contact solar cell, the method comprising:depositing spaced-apart first and second pads of a material containingboron onto the planar rear surface of a semiconductor substrate overfirst and second substrate regions, respectively; heating the substratesuch that a first portion of the boron diffuses from the first andsecond pads into said first and second substrate regions, respectively,through said planar rear surface to form first and second diffusionregions, and a second portion of the boron forming the first and secondpads is converted into first and second borosilicate glass diffusionbarrier structures, respectively, that are formed on said planar rearsurface over said first and second diffusion regions during saidheating, and after said spaced-apart first and second borosilicate glassdiffusion barrier structures are formed, diffusing a second dopant intoa third substrate region through the planar rear surface of thesemiconductor substrate between said first and second borosilicate glassdiffusion barrier structures to form a third diffusion region such thatsaid second dopant disposed in the third diffusion region forms a firstinterface with the first dopant disposed in the first diffusion regionand a second interface with the first dopant disposed in the seconddiffusion region, wherein said first and second borosilicate glassdiffusion barrier structures are formed prior to said second dopantdiffusion such that said first and second borosilicate glass diffusionbarrier structures prevent diffusion of said second dopant into saidfirst and second substrate regions during said second dopant diffusion.5. The method according to claim 4, wherein diffusing the second dopantcomprises, after forming said first and second borosilicate glassdiffusion barrier structures, disposing said substrate in a furnacecontaining POCl₃ at a temperature selected such that phosphorus isdiffused into said third substrate region.
 6. The method according toclaim 4, further comprising forming a plurality of grooves into theplanar rear surface of the substrate such that the first diffusionregion is separated from the third diffusion region by a first groove,and the second diffusion region is separated from the third diffusionregion by a second groove.
 7. A method of fabricating an interdigitatedback contact solar cell, the method comprising: depositing spaced-apartpads of a material containing boron onto a planar rear surface of asemiconductor substrate over first and second substrate regions; heatingthe substrate such that a first portion of the boron diffuses into saidfirst and second substrate regions through said planar rear surface toform first and second diffusion regions, and such that a second portionof the boron forms first and second borosilicate glass structures onsaid planar rear surface over said first and second diffusion regions,and after said first and second borosilicate glass structures areformed, diffusing a second dopant through the planar rear surface of thesemiconductor substrate into a third substrate region to form a thirddiffusion region such that said second dopant disposed in the thirddiffusion region forms a first interface with the first dopant disposedin the first diffusion region and a second interface with the firstdopant disposed in the second diffusion region, wherein said heating isperformed such that said first and second borosilicate glass structuresform effective barriers that prevent diffusion of said second dopantinto said first and second substrate regions during said second dopantdiffusion.
 8. The method according to claim 7, wherein diffusing saidsecond dopant comprises disposing said substrate in a furnace containingPOCl₃ at a temperature selected such that phosphorus is diffused intosaid third substrate region and said glass structures prevent diffusionof Phosphorous from said POCl₃ into said first and second substrateregions.
 9. The method according to claim 7, further comprising forminga plurality of grooves into the planar rear surface of the substratesuch that the first diffusion region is separated from the thirddiffusion region by a first groove, and the second diffusion region isseparated from the third diffusion region by a second groove.